1. Industrial Field of the Invention
The present invention relates to a magnetic linear encoder for detecting a linear velocity or distance in a state of not being brought into contact with a magnetic resistance element or a magnetic rotary encoder for detecting a rotational angle or velocity in the same state, as well as a magnetic recorder for use in such a magnetic encoder.
2. Description of the Related Art
Magnetic encoders (magneto-electric converter) which employ a magneto resistance effect element made of a thin ferromagnetic film, have been commonly used in various fields due to their good durability in a surrounding atmosphere, wide operational temperature range and high response frequency. For example, magnetic encoder is used for controlling the rotational speed of a capstan motor in a VTR (video tape recorder) or the like. Generally speaking, magnetic encoders are used for positional or speed control in factory automatic (FA) equipments, such as servomotors, robots and NC machine tools, or in office automation (OA) equipments, such as computers and copying machines. In recent years, there has been an increasing demand for improving the accuracy of such equipments and intensive studies have thus been done to develop a magnetic encoder having with high resolution.
Conventional magnetic encoders are classified into a rotary type shown in FIG. 7A and a linear type shown in FIG. 7B. The magnetic encoder of either type includes a magnetic recorder 1 and a magnetic sensor 2 disposed in opposition to the magnetic recorder 1. The magnetic recorder 1 comprises a non-magnetic substrate 11 and a recording medium 12 which is a permanent magnetic material coated on the peripheral or flat surface of the non-magnetic substrate 11. The recording medium 12 is magnetized in a multipolar fashion at a magnetizing pitch .lambda. to form at least one magnetic signal track.
On the magnetic sensor 2, a plurality of magnetic resistance elements (hereinafter referred to MR elements) 22 with stripe are formed substantially parallel to the boundary lines between the adjacent signals on the magnetic signal track. The magnetic resistance elements are formed by processing a thin film of an Fe-Ni, Ni-Co or Fe-Ni-Co alloy coated on the surface of a non-magnetic substrate 21 using a photo-lithographic method.
When the magnetic signal track and the magnetic sensor 2 (disposed in opposed relation to the magnetic signal track) move relative to each other, the MR elements of the magnetic sensor are subjected to magnetic fields which change in an alternative fashion, and the resistance of each of the MR elements is changed synchronously with changes in the magnetic fields. To convert these changes in the resistance into an electric signal, a bridge structure may be provided in which a pair of MR elements are disposed at an interval of .lambda./2 in order to produce a differential output voltage. The recording medium 12 is formed by coating on the surface of a non-magnetic substrate a solution obtained by diluting a material composed of 60 to 70 wt. % of acicular magnetic particles and 40 to 30 wt. % of binder, such as an epoxy or polyurethane resin, in a solvent and then by drying the coated film. The acicular magnetic particles may be .gamma.-Fe.sub.2 O.sub.3, Co(cobalt)-.gamma.Fe.sub.2 O.sub.3 or other metal magnetic powders used in conventional magnetic recorder, such as in hard disks, floppy disks or VTRs.
It is noted that encoders are not always used under ideal environments. Among the above-described magnetic powders, Co-.gamma.Fe.sub.2 O.sub.3 has excellent durability and magnetic characteristics and is therefore widely used. Co-.gamma.Fe.sub.2 O.sub.3 magnetic powder is composed of acicular particles each of which has a saturation magnetization .sigma.s of 60 to 80 emu/g, a coercive force Hc of 250 to 1000 Oe, a particle length of 0.2 to 0.8 .mu.m and an axial ratio of 5 to 10.
As the recording medium film becomes thinner, the diamagnetizing field becomes weaker, thus lessening demagnetization of the recording medium and generating a more effective magnetic force. However, as the film becomes thinner, the volume of the magnet is reduced, thereby reducing the generated energy. Therefore, presently the thickness of the film is set to about half of or is made substantially equal to the magnetizing pitch .lambda.. For example, a recording medium having a film thickness of 80 to 100 .mu.m is used relative to the magnetizing pitch .lambda. of 125 .mu.m.
FIG. 8 shows a relationship between the space (or spacing) between a magnetic drum (the magnetic recorder of a rotary encoder) and MR elements and an output voltage. In this case, the magnetic drum has a 80 .mu.m thick recording film made of Co-.gamma.Fe.sub.2 O.sub.3 having a residual magnetic flux density (Br) or 2,130 Gauss, a coercive force (Hc) of 800 oersted, and which is magnetized in a multipolar fashion by a pitch of 125 .mu.m. According to the graph of FIG. 8, the range of the space which ensures the maximum output is between 60 to 80 .mu.m, which is about one half of the magnetizing pitch 125 .mu.m (see JP-A-58-117411 and JP-A-59-28220).
According to the aforementioned relationship between the space between the magnetic recording medium (made of Co-.gamma.Fe.sub.2 O.sub.3) and the MR elements and the output voltage, the space which ensures the maximum output voltage is about one half of the magnetizing pitch, and the range of the space which can generate the maximum output voltage is narrow. When it is desired to further reduce the magnetizing pitch, the space has to be reduced greatly, which may be difficult and cause contact between the recording member and the magnetic sensor during the operation of the magnetic encoder.
To overcome this problem, increasing the proportion of the magnetic powder in a coating film to improve the magnetic characteristics of the coating film has been considered. However, the present inventors noted when the coating material containing a high proportion of acicular magnetic powder, such as Co-.gamma.Fe.sub.2 O.sub.3, and a resin binder is thickly coated, cracks are readily generated during heating. The cracks in the recording medium are undesirable because they generate noise in an output signal or decrease the peak value. Therefore, when the acicular magnetic powder, such as Co-.gamma.Fe.sub.2 O.sub.3, is used to form a 80 to 100 .mu.m thick magnetic film, the proportion of the magnetic powder in the solid content of the magnetic film should be limited to between 60 and 70 wt. % (25 and 35 vol. %). There is a limit to the improvement in the magnetic characteristics achieved by an increase in the proportion of the magnetic powder. A magnetic recording medium which contains 65 wt. % (30.3 vol. %) of Co-.gamma.Fe.sub.2 O.sub.3 magnetic powder exhibits a residual magnetic flux density Br of about 941 G because it is not magnetically oriented.